1. Field of the Invention
The present invention relates to remote meter reading systems of the type which use transponders fitted to the meters and an interrogate/receiver instrument which sends out a common "wake up" signal to activate a group of transponders within range.
2. Description of Related Art
Examples of this type of meter reading system are described in U.S. Pat. No. 4,614,945 and U.S. Pat. No. 4,799,059 (ENSCAN).
In such remote meter reading systems the transponders are designed to transmit a data packet to the interrogate/receiver instrument when activated by the wake up signal. These data packets contain data identifying the meter number and readings. With any such system it is necessary to provide a solution to the technical problem of possible collision between the data packets transmitted by the transponders within the activated group.
In the ENSCAN patents described above, collision avoidance is provided by the transponders having a transmitter which has means of varying its transmission frequency in accordance with a pre-determined function. In this way, the data packet from the activated transponders may occur at different frequencies within a pre-determined frequency bandwidth. The ENSCAN patents also provide for repeated transmission of the data packet to assist collision avoidance.
The ENSCAN patents therefore disclose one example of spread spectrum modulation. However, this system requires the receiver instrument to have the ability to listen simultaneously on all possible channels on which the transponders may transmit. The collision avoidance technique is not guaranteed to succeed. There is only a statistical chance that the transmissions from the group of activated transponders will be received without collision. Repeated transmission is therefore also necessary.
Spread spectrum technology has been used in the field of radio communications to eliminate the effects of interference and to prevent eavesdropping. For a fuller description of spread spectrum technology and its implementation in communication systems, reference may be made to Spread Spectrum Systems, 2nd Edition by Robert C. Dixon, published by Wiley Intersciences (ISBN 0-471-88309-3) or Coherent Spread Spectrum Systems by Jack K. Holmes also published by Wiley Interscience (ISBN 0-471-03301-4).
As well as the frequency hopping type of spread spectrum technology used in ENSCAN, communications systems can use a technique known as direct sequence spread spectrum. In this technique the carrier is multiplied by a high speed psuedo-random digital code signal. A receiver provided with the same psuedo-random digital code signal can de-spread the received signal to regenerate the original carrier. In order to achieve regeneration it is necessary for the pseudo-random codes used by the transmitter and receiver to be locked together. This synchronization step can be obtained by a number of techniques. Such techniques, however, all take a significant time to achieve lock. In a typical application which uses a 127 chip maximum length pseudo-random code signal with an "early/late" delay lock loop it may take several seconds to achieve lock. This delay would make these techniques unsuitable for use in remote meter reading systems as it would prevent rapid reception of the data packet. Since transponders are normally battery powered, long transmissions must be avoided to reduce battery consumption. Remote meter reading systems may also require the interrogate/receiver instrument to be a vehicle which is driven past the meters to be read. This further limits the time during which the receiver is within range in order to complete the process.